A Minnesota police officer, whose fatal shooting of a black motorist transfixed the nation when his girlfriend livestreamed the aftermath, was acquitted of all charges on Friday.

The officer, Jeronimo Yanez, had been charged with second-degree manslaughter and endangering safety by discharging a firearm in the shooting of Philando Castile.

After the verdict, jurors and Mr. Yanez were quickly led out of the courtroom, and Mr. Castile’s family left immediately. When a deputy tried to stop his mother, Valerie, she yelled “Let me go.”

Later, she said: “My son loved this city, and this city killed my son. And a murderer gets away. Are you kidding me right now?”

She continued: “The system in this country continues to fail black people and will continue to fail us.”

A handful of protesters gathered outside the Ramsey County courthouse. “It’s not us that were on trial, it was the system that was on trial,” Mel Reeves, a community activist, said. “Yanez worked for the system. He killed somebody, right. Philando Castile got victimized by the system.”

The verdict, which came on the fifth day of deliberations, followed the acquittal last month of an Oklahoma police officer who killed an unarmed black man and, of particular interest here, a prosecutor’s decision last year not to charge the Minneapolis officers who killed another black man, Jamar Clark.